First Observations of the Surface of Two Oort Cloud Objects
Negative images of C/2013 P2, obtained with the Gemini North 8-meter telescope, and of C/2014 S3, obtained using the CFHT. Both images have been processed to remove most of the background stars and galaxies to enhance the visibility of the faint dust tail. The tail in the left image is the very faint extension toward the 10 o’clock position. In both images, the icy body producing the dust is not resolved, and the central dark region appears larger both because of the effect of telescope optics and because of the dust cloud close to the bodies.
A team led by IfA astronomer Karen Meech has discovered two unusual objects that originate in the Oort Cloud, a spherical halo of comet nuclei in the outer solar system that extends to about 100,000 times the Earth-Sun distance. Although their orbits are like those of Oort-Cloud comets, these objects show almost no comet-like activity, allowing scientists to see their surfaces. These results are particularly intriguing because the objects’ surfaces are different from what astronomers expected, and the objects give them clues about the movement of material in the early solar system as the planets were assembled.
In August 2013, when the Pan STARRS1 survey telescope (PS1) on Haleakalā discovered the first object, C/2013 P2 Pan-STARRS, it looked like an asteroid. “Objects on long-period orbits like this usually exhibit cometary tails, for example comet ISON and comet Hale Bopp, so we immediately knew this object was unusual,” explained Meech. “I wondered if this could be the first evidence of movement of solar system building blocks from the inner solar system to the Oort Cloud.”
Follow-up observations in September 2013 with the 8-meter Gemini North telescope on Maunakea hinted at faint, low-level light reflected off a dusty tail. This tail remained through the object’s closest approach to the Sun (2.8 times the Earth-Sun distance, within the outer asteroid belt) in February 2014, but the object didn’t get much brighter. A spectrum of the surface obtained with Gemini North in the spring of 2014 showed that the object was very red, completely different from comet or asteroid surfaces, and more like the surface of an ultra-red Kuiper Belt object. The Kuiper Belt is a region that lies just beyond the orbit of Neptune.
“We had never seen a naked (inactive) Oort Cloud comet, but Jan Oort hypothesized their existence back in 1950 when he inferred the existence of what we now call the Oort Cloud. Oort suggested that these bodies might have a layer of ‘volatile frosting’ left over from 4.5 billion years of space radiation that disappears after their first pass through the inner solar system. Maybe we are seeing the first evidence of this,” said team member Olivier Hainaut of the European Southern Observatory.
While the team analyzed their observations of comet C/2013 P2 Pan-STARRS, a second object was discovered. C/2014 S3 Pan-STARRS was discovered through the NASA-sponsored Near Earth Object Survey on the PS1 telescope on September 22, 2014. Like C/2013 P2 Pan-STARRS, it also had an Oort Cloud–type cometary orbit and showed minimal activity. To the team’s surprise, follow-up observations with the Canada-France-Hawaii Telescope (CFHT) on Maunakea showed that this object has colors similar to inner solar system asteroids, the first outer solar system object to do so. While the orbit of C/2014 S3 Pan-STARRS took it closer to the Sun than C/2013 P2 (between the asteroid belt and the orbit of Mars), it also barely had a tail.
“I’ll be thrilled if this object turns out to have a surface composition similar to asteroids in the inner part of the asteroid belt. If this is the case, it will be remarkable for a body found so far out in the solar system, especially since it exhibited a tail that may be due to volatile outgassing,” commented Meech. “There are several models that try to explain how the planets grew in the early solar system, and some of these predict that material formed close to the Sun could have been thrown outward into the outer solar system and Oort Cloud, where it remains today. Maybe we are finally seeing that evidence.”